This invention relates generally to an apparatus and method for storing electrograms and more particularly to the implementation of such a device in the context of an implantable cardioverter/defibrillator.
Implantable cardioverter/defibrillators and pacemakers have for some time included the capability of storing digitized cardiac electrograms for diagnostic purposes. Most commonly, such electrograms are stored in conjunction with the detection of cardiac arrhythmias, and include the period of time immediately preceding detection of the arrhythmia. For example, the device disclosed in U.S. Pat. No. 4,223,678 issued to Langer et al and the device disclosed in U.S. Pat. No. 4,295,474 issued to Fischell, both employ looping memories to continually store cardiac electrograms, with the contents of the looping memory being frozen on occurrence of an arrhythmic event and transferred to permanent storage. The devices also store electrograms following either detection of the arrhythmic event or delivery of an anti-arrhythmic therapy. While this approach to electrogram storage does provide the physician with information associated with the detected arrhythmia and its treatment, it has the drawback of requiring continuous operation of the looping memory and the sense amplifier, increasing the current drain on the battery and shortening the potential lifespan of the device.
Prior approaches to reducing the current drain associated with electrogram storage have focused on the storage of markers indicative of heart paced or sensed heart depolarizations and/or intervals between depolarizations and on design of low power sense amplifiers. While these approaches have been somewhat successful, the information stored in the form of markers or interval is not always adequate for diagnostic purposes and continuous activation of even low power sense amplifiers in conjunction with looping memories still results in an undesirably high current drain.